1. Field of the Invention
The present invention relates to a photo-electric converter which varies an accumulation time as an illumination of an object changes so that an output voltage in a predetermined range is always produced.
2. Related Background Art
Since the illumination of the object varies in a wide range such as 10.sup.-3 to 10.sup.3 luxes, a sufficient dynamic range of the output voltage in a prior art photo-electric converter used for AF (auto-focusing) means of a camera cannot be attained by setting a fixed accumulation time. Thus, the accumulation time is varied with the change of the illumination of the object and the output voltage is amplified in accordance with the accumulation time to attain a so-called AGC (auto-gain control) so that an output voltage within a predetermined range is produced without regard to the illumination of the object.
In such a contrast control type photo-electric converter, a control method for detecting the illumination of the object on real-time basis and terminating the accumulation when a predetermined input level is reached is disclosed in Japanese Laid-Open Patent Application No. 61-167916.
In an improvement over the above control method, maximum and minimum values of the illumination are detected on real-time basis, and the accumulation time is changed in accordance with a difference between the maximum and the minimum, that is, a contrast of the object, and the output voltage is amplified in accordance with the contrast, is disclosed in Japanese Laid-Open Patent Application No. 1-222583.
In the disclosed construction, a constant contrast component can always be taken out without regard to the condition of the object so that a control system which is strong to a low contrast can be attained.
FIG. 1 shows a circuit block diagram of a configuration of a photo-electric conversion sensor unit and a control unit of a prior art contrast control type photo-electric converter. Numerals 401-401n denote NPN transistors which function as photo-sensing elements, numerals 402-402n denote switching transistors for resetting base electrodes of the NPN transistors 401-401n, numerals 403-403n denote switching transistors for resetting emitter electrodes of the transistors 401-401n, numerals 404-404n denote capacitors for storing output voltages of the transistors 401-401n, numerals 405-405n denote switching transistors for transferring the outputs of the transistors 401-401n which function as the photo-sensing elements to the capacitors 404-404n for controlling the accumulation time, numerals 406-406n denote switching transistors for reading out the voltages stored in the capacitors 404-404n, numeral 408 denotes a common output line to which the outputs are read, and numeral 409 denotes a stray capacity of the common output line 408.
Numeral 410 denotes a shift register for scanning bits, numeral 411 denotes an output buffer, numeral 412 denotes an amplifier for real-time monitoring a maximum value of the outputs of the photo-sensing transistors, numeral 413 denotes an amplifier for real-time monitoring a minimum value of the outputs of the photo-sensing transistors 401-401n, numeral 414 denotes an output buffer, numeral 415 denotes a differential amplifier, numerals 416, 417 and 418 denote resistors for producing desired voltages, numerals 419, 420 and 421 denote comparators, numeral 422 denotes a control circuit which receives the outputs of the comparators 419, 420 and 422 to drive the switching transistors 405-405n for controlling the accmulation time, numeral 423 denotes a reference voltage terminal for determining a comparison reference voltage of the comparators, numeral 424 denotes a photo-electric conversion sensor output terminal, numeral 425 denotes an accumulation time control pulse output terminal, numeral 426 denotes a sensor collector power supply terminal, numeral 427 denotes a control terminal for the switching transistors 402-402n, numeral 428 denotes a base reset terminal for the switching transistors 402-402n, numeral 429 denotes a control terminal for the switching transistors 403-403n, and numeral 430 denotes an emitter reset power supply terminal for the switching transistors 403-403n.
FIG. 2 shows a timing chart of a photo-sensor reset period and an accumulation time in the prior art configuration.
When pulses are applied to the terminals 425, 427 and 429, the output of the differential amplifier 415 gradually rises on real time basis from the start of accumulation in accordance with the contrast of the object, as shown in FIG. 2. Reference voltages of the comparators 419, 420 and 421 for comparing the outputs are referred to as A, B and C, and the prior art control method is explained below.
FIG. 3 illustrates the control of the output voltage versus the accumulation time for explaining an operation of the prior art photo-electric converter.
The output of the differential amplifier 415 shown in FIG. 1, that is, the output of the real time contrast monitor rises with the accumulation time. A time point T.sub.B is set in the accumulation time period and following five controls are conducted.
1 When the output exceeds the level A before the accumulation time T.sub.B, the accumulation is terminated at that time point t.sub.1.
2 When the output exceeds the level B at the time point T.sub.B, the accumulation is continued until the output reaches the level A, when the accumulation is terminated at that time point t.sub.2.
3 When the output exceeds the level C at the time point T.sub.B, the accumulation is continued until the output reaches the level B, when the accumulation is terminated at that time point t.sub.3.
4 When the output is below the level C at the time point T.sub.B, the accumulation is continued until the output reaches the level C, when the accumulation is terminated at that time point t.sub.4.
5 When the output does not reach the level C before a preset maximum accumulation time T.sub.MAX, the accumulation is terminated at the time point T.sub.MAX.
The output of the photo-electric conversion sensor is multiplied by the gain corresponding to the level A, B or C so that proper AGC is attained and the output in the predetermined range is attained without regard to the contrast of the object.
The outputs of the transistors 401-401n which function as the photo-sensors are temporarily transferred to the accumulation capacitors 404-404n through the transistors 405-405n at the end of accumulation. Then, the shift register 410 is serially scanned to read out the content thereof to the common line 408 through the transistors 406-406n. The output V.sub.out in given by EQU V.sub.out =V.sub.E .multidot.C.sub.T /(C.sub.T +C.sub.H)
where C.sub.T is the capacitance of the accumulation capacitor 404, C.sub.H is the stray capacitance 409 of the common line 408 and V.sub.E is a voltage at C.sub.T.
Main components of C.sub.H are a drain capacitance of the transistor 406, a wiring capacitance of the common line 408 and an input capacitance of the output buffer 411. A gain determined by the level of the comparator is multiplied to V.sub.out in a succeeding stage processing circuit.
However, in a recent AF system, a plurality of columns of photo-sensing elements are formed in one chip, different numbers of pixels are used, or different orientations are used in order to enhance the functions.
Where a plurality of sensor columns each including a plurality of photo-sensing elements are arranged and the number of pixels of the sensor columns are different from each other, the numbers of reading switching transistors 406 are different from each other in the prior art configuration. As a result, the capacitances C.sub.H of the stray capacitors 409 are different from sensor column to sensor column.
Where the photo-sensing elements are not arranged in one direction, the wiring lengths of the common line 408 are different from sensor column to sensor column. This also causes different values of C.sub.H. The fact that the values of C.sub.H are different from sensor column to sensor column means that the output voltages from the respective sensor columns are different. This causes ununiformity in sensitivity.
If the AGC is applied to such a plurality of sensor columns by the same comparator level, the ununiformity in the sensitivity due to the difference in C.sub.H is directly amplified.